(1) Field of the Invention
This invention relates to a process for preparing a lactone, especially .gamma.-butyrolactone.
.gamma.-Butyrolactone is useful, for example, as a raw material for pyrrolidones and as a solvent for the preparation of an electrically conductive solution.
(2) Description of the Related Art
Heretofore, many proposals have been made as to the processes for preparing lactones by catalytic hydrogenation of saturated or unsaturated dicarboxylic acids or functional derivatives thereof in the liquid phase. For example, processes have been proposed which comprise using as the hydrogenation catalyst a palladium catalyst supported on active carbon (U.S. Pat. No. 3,113,138), a nickel-containing catalyst (Japanese Examined Patent Publication No. 43-6947), and a cobalt-palladium catalyst (Japanes Examined Patent Publication No. 58-29142).
In the process using a palladium catalyst supported on active carbon, when succinic anhydride is used as the raw material, .gamma.-butyrolactone can be obtained in a high yield, but when maleic anhydride is used as the raw material, two steps are indispensable for the hydrogenation reaction and further the catalyst must be added in the midst of reaction. The process using a nickel-containing catalyst or a cobalt-palladium catalyst are advantageous from a viewpoint of the catalyst cost, but severe reaction conditions, e.g., a temperature of 250.degree. C. and a pressure of 100 kg/cm.sup.2, are usually necessary and therefore, undesirable side reactions occur which include, for example, production of cyclic ethers and decarbonilation, and selectivities to lactones are low.